fresh gas flow rates

heartICU · Oct 6, 2005

I have a couple of questions about gas flow rates:
1. What are the typical flow rates for oxygen, nitrous and air in the anesthesia machine?
2. When and why would you use air instead of oxygen?
3. How much oxygen do you need to run with nitrous?
Thank you very much,
Mark

1. Depends on what you are trying to do. If you are trying to get some agent on quickly or get it down quickly, high flow rates will help you do that. Low flow rates will help you preserve existing gas that's in, or slowly bring on new agent. Lower rates will also cause you to use less agent (for example, 6% Des when you are running flow rates of 10L will use up a lot more des than if you are running it with flow rates of 2L). This translates into less cost for the department and for the patient. I usually run my flows at 1L of air and 0.7L of O2, or 1L N2O and 0.7L O2, unless its sevo. Running low flow will also keep the patient warm.

2. In most cases, I would not use air instead of oxygen. I use air a lot of times instead of nitrous. I can think of only once that I ran someone on air and nothing else - it was a radical retro prostatectomy, with periaortic lymph node dissections in a patient with a history of testicular cancer. The patient had been on bleomycin, and bleo can cause pulmonary fibrosis if you expose lungs to high FI02. That case I ran on 2L of air the whole time, no problems. You can also run your patient on air if you lose wall pressure - of course, switch to your O2 tank first, but when that runs out, bag 'em on room air.

I guess you would want to also use low FI02s with laser procedures to prevent airway fire (in ENT). I haven't done my ENT rotation yet, so maybe someone who has more experience with laser tumor destructions in the airway can shed some light.

3. When you run oxygen with nitrious, the bare minimum you need to give is the same as atmospheric air: 21%. Anything less will give you a hypoxic mixture. All machines made now have built in safety mechanisms to prevent the anesthetist from delivering a hypoxic mixture, but you still need to know how to calculate the FI02 in case those safety devices fail.

MaleAPRN · Oct 12, 2005

Do you know how to calculate nitrous and O2 mixture on paper? Without looking at the expired number given out by the anesthesia machine? Thanks.

heartICU · Oct 13, 2005

Do you know how to calculate nitrous and O2 mixture on paper? Without looking at the expired number given out by the anesthesia machine? Thanks.

Yes. Take your flow of oxygen and divide that by your total gas flow. For example...

If your total gas flow is 3L (2L N2O and 1L O2) then to calculate your nitrous total, I divide the total amount of nitrous by the total gas flow. 2divided by three gives you 0.67, so you are running 67% nitrous. To calculate your O2 total, divide the total amount of oxygen by the total gas flow. 1L/3L = 0.33, so you are running 33% oxygen. This is easy when you are doing nitrous and oxygen. It is harder when you throw in air, or if you are running nitrous, air and oxygen at the same time.

When I calculate it if I have air in the equation, I do it by 1000. I am not sure if this is going to make sense as I explain it on this bulletin board, but here goes. If you have a total gas flow of 3L (2L air and 1L O2) I equate each liter to 1000ml. Then you divide the same way. To calculate the percentage of oxygen, take the total amount of oxygen (which is going to be 100% of the third liter or 1000ml, and 21% of each of the liters of air, or 210 ml each, for a total of 1420) and divide it by the total gas flow (3000ml). So 1420/3000 is 0.47, or 47% FIO2. Make sense?

jwk · Oct 13, 2005

When I calculate it if I have air in the equation, I do it by 1000. I am not sure if this is going to make sense as I explain it on this bulletin board, but here goes. If you have a total gas flow of 3L (2L air and 1L O2) I equate each liter to 1000ml. Then you divide the same way. To calculate the percentage of oxygen, take the total amount of oxygen (which is going to be 100% of the third liter or 1000ml, and 21% of each of the liters of air, or 210 ml each, for a total of 1420) and divide it by the total gas flow (3000ml). So 1420/3000 is 0.47, or 47% FIO2. Make sense?

Here's an old trick.

O2=100%, Air=20% (just to use a round number)

O2 100 20

\ /

40 20:60 = 1:3 ratio of O2 to Air

/ \

AIR 20 60

Put your desired O2 %age in the middle

Going diagonally, the difference between 100 and 40 is 60.

The difference between 20 and 40 is 20.

You see 20 and 60 for the two right-hand numbers. That is your ratio of O2 to Air. 20:60 = 1:3, so 1 liter of O2 and 3 liters of air gives an approximate O2 concentration of 40%.

100 40

\ /

60

/ \

20 40

40:40 = 1:1 1 liter O2 and 1 liter of Air gives you a 60% O2 concentration.

This works for any percentage you want to use to figure out your gas flows.

(Sorry, my graphical interpretation doesn't come through well, but hopefully you get the idea)

mwbeah · Oct 19, 2005

Just another technique

1 liter of air contains roughly 210 ml O2

1 liter of O2 contains 1000 ml O2

1 liter of N2O contain 0 ml O2

example using air

So 2 Liter of air and 1 liter of O2

- 2 * 210 = 420

- 1 * 1000 = 1000

1420/3000 = .473 (47% O2)

Laughing Gas · Oct 24, 2005

The new narkomeds have a mode that allows you to run an FIO2 of 18%

FYI

heartICU · Oct 24, 2005

The new narkomeds have a mode that allows you to run an FIO2 of 18%
FYI

What are your gas flow rates? What is the purpose of running a hypoxic mixture?

goodgrief · Dec 1, 2010

If you need to calculate 45% FiO2 with 3000 Total Flow using O2/Air combination:

First calculate Total O2 Content

Total O2 Content = 0.45 * 3000

Total O2 Content = 1350

Second calculate for air

(Total Flow - Total O2 content) / 0.79

Air = (3000 - 1350) / 0.79

Air = 1650 / 0.79

Air = 2088.61 or 2100mL or 2.1 L

So, set your Air flow meter to 2.1L

Total Flow - Air = O2 Flow

O2 Flow = 3000 - 2100

O2 Flow = 900mL or 0.9L